1. Technical Field of the Invention
The present invention relates generally to a semiconductor device having an insulating film, and a method for fabricating the same.
2. Description of Related Art
Since a typical semiconductor oxide film has a very great part as an insulating film in various semiconductor devices, the qualities and forming methods thereof have been variously studied. As a method for forming a semiconductor oxide film, a so-called thermal oxidation process for exposing the surface of a semiconductor to oxygen molecular gas in the atmosphere at high temperatures has been widely used. With the scale down of elements, it is considered that the thickness of the thermal oxide film is decreasing. However, if the thickness of the oxide film is 2 nm or less, the current tunneling through the oxide film rapidly increases to cause a phenomenon that impurities pass through the oxide film to diffuse. For that reason, it is being difficult to improve the performance of elements due to the scale down.
Therefore, there is considered a method for mixing nitrogen in an oxide film to form an oxynitride film. If nitrogen atoms are introduced into an oxide film, the dielectric constant of the oxynitride film increases, so that the thickness of an oxynitride film having the same capacitance as that of an oxide film can be larger. In addition, since the diffusion of impurities, such as boron, can be suppressed, it has been possible to effectively form a thinner high-performance insulating film by the conversion to an oxide film.
However, if nitrogen atoms are introduced into an oxide film, energy levels due to nitrogen atoms are formed in a band gap in the insulating film although the effective thickness can be decreased. For that reason, if an oxide film into which nitrogen atoms are introduced is used as, e.g., a gate insulating film for a MOS transistor, current drivability decreases due to a degradation of carrier mobility. In order to prevent this, there is considered a method for preventing the scattering of electrons by localizing introduced nitrogen atoms in the vicinity of the surface of the gate insulating film so as to be spaced from the interface between the semiconductor layer and the gate insulating film. However, it is difficult for this method to completely control the doping amount and to reduce the energy level due to nitrogen atoms.
Japanese Patent Laid-Open Publication No. 2001-203198 discloses a method for forming an oxynitride film. In this method, the surface of a silicon substrate is hydrogen-terminated, and hydrogen atoms are removed by heat treatment. Thereafter, nitrogen atoms and oxygen atoms are absorbed onto unbonded bonds in a heating atmosphere of NO gas or NO+O2 to form a monoatomic oxynitride layer. Thereafter, it is oxidized in the atmosphere to form an oxynitride film having an oxide layer on the side of the silicon substrate and an oxynitride layer on the side of the surface. However, most of nitrogen atoms in the oxynitride film formed by this method are in a two-coordinate bond state, so that it is difficult to reduce the energy level due to nitrogen atoms in the band gap of the oxynitride film.